This invention relates to an aluminum nitride substrate and to a method for producing it.
The field of applicability of ceramics has increased in recent years, and electronic components have come to occupy an important position within it. Semiconductor substrates have become an important application. In fact, notwithstanding its recent development to the stage of practical implementation, the technology of applying aluminum nitride ceramics as semiconductor element substrates is still the subject of vigorous research concerning detailed aspects thereof.
One of them is a method of producing many products at the same time. A mass-production method for aluminum nitride ceramics has been considered. There are many uses for the substrate. For purposes of illustration, however, a semiconductor substrate which has a size of 10 mm.times.10 mm.times.0.7 mm will be discussed.
A method of producing semiconductor substrates includes several steps of preparing a ceramic substrate, metallizing onto the substrate, electroless-plating onto the metallized portion, mounting a silicon semiconductor chip thereon, electrically connecting the chip with the electroless-plated portion through bonding wires, covering the substrate with resin, and installation into a product of transistor modules. It has been noticed that it is not advantageous to treat substrates one by one according to these steps. This is because the substrates are too small to be treated in these steps. Separate processing results in a ratio of good products to total products that becomes worse as the substrates become smaller.
A method has been developed for using a large ceramic substrate. That is, the method includes several steps of preparing a large ceramic substrate, metallizing many patterns onto the substrate, electroless-plating onto the metallized portions, sub-dividing the substrate into prescribed dimensions, mounting silicon semiconductor chips thereon, respectively, electrically connecting each chip with the electroless-plated portion through bonding wires, covering the substrate with resin, and installation into products of transistor modules. In this case, a high density energy means was used only by the applicant of the present invention for sub-dividing the substrate into prescribed dimensions. For example, there are laser method and electron beam means etc. as a high density energy means. In this case, laser method is better as a high density energy means because its equipment is simpler and cheaper than that of electron beam means. This method of using a laser has two steps, namely, making some holes, consecutive holes or grooves (hereinafter, these are called score lines) on the substrate by directing the laser at prescribed portions on the substrate, and severing the substrate along these score lines after the step of electroless-plating.
When score lines are formed in this way on an aluminum nitride substrate surface by high density energy working, the phenomenon of segregation of metallic aluminum in the vicinity of this worked surface has been found to occur. It is therefore found that, in the nickel plating stage, in which the substrate is covered with a metallizing layer constituting the electrically conductive layer necessary for an electronic component, the plating layer also covers the metallic layer that is present in the worked surface. This impairs the withstand-voltage characteristic that is required for a substrate to be used for electronic components.
Several methods have been developed for removing this metallic aluminum by the applicant of the present invention. They employ mechanical means in a honing step, using so-called abrasive etc., or placing it in water and irradiating with a laser. However, in the case of the former method, it is difficult to verify whether or not removal is complete. And in the latter method, the drawback that complicated system is required cannot be avoided.